8150( 4) - 02/05/99 16:16 - [ 1] PROPOSAL FOR HUBBLE SPACE TELESCOPE OBSERVATIONS ST ScI Use Only ID: 8150 Version: 4 Check-in Date: 05-Feb-1999 16:15:21 1.Proposal Title: The metallicity and dust content of HVC complex C ------------------------------------------------------------------------------------ 2. Proposal For 3. Cycle GO 8 ------------------------------------------------------------------------------------ 4. Investigators Contact? PI: Bart Wakker University of Wisconsin CoI: Dr. Blair Savage University of Wisconsin-Madison N ------------------------------------------------------------------------------------ 5. Abstract From observations of S II, H I and HAlpha toward the Seyfert galaxy Mark 290, we have recently determined that the high- velocity cloud complex C has a metallicity ~0.07+/-0.02 times solar. Other work shows that this extensive cloud complex is at least 3 kpc above the galactic plane and probably closer than 30 kpc, placing it in the Galactic halo. These results make it likely that complex C is a remnant of the formation of the Galaxy or Local Group. It may represent a present-day analogue of the damped LyAlpha absorbers seen in QSO spectra. Our abundance result provides the first direct observational evidence for the infall of low-metallicity gas on the Milky Way, required in models of galactic chemical evolution. We request time to obtain the near-UV spectrum of Mark 290, to measure column densities for Zn, Fe, Mn and Cr. Zn and S are produced by different nucleosynthetic pathways (S is an Alpha element, Zn an Fe peak element) and both are usually only slightly depleted onto dust; thus, the S/Zn ratio provides crucial information about the element production history of complex C. Fe, Mn and Cr are usually mostly in dust grains, so their ratios with S and Zn will tell us about the presence, amount and properties of the dust. These observations will provide important zero-redshift abundance information for the study of gas and dust in damped LyAlpha systems. ------------------------------------------------------------------------------------ 8150( 4) - 02/05/99 16:16 - [ 2] Observations Description ------------------------ First, we discuss the feasibility of the proposed program. We will use the E230M echelle, set up for a central wavelength of 2269 Angstrom, covering the wavelength range 1865--2673 Angstrom, to take one spectrum of Mark 290. Optical depths for the lines of interest can be predicted in the following manner. We use standard solar abundances (Anders & Grevesse 1989) scaled down by a factor 9, since we observed that ([S II]/\HI)~0.11 (the final value of S/H=0.07 includes a correction for H^+). We assume that Zn is not depleted onto dust, and used halo depletions (relative to Zn) for Cr, Mn and Fe (Savage & Sembach 1996). Atomic data are from Verner et al.\ (1994). The \HI\ column density is found from the Effelsberg profile as 7tdex19 cm^-2 (see Fig. 2). We further assume an absorption linewidth of 15 km/s as observed for \CaII\ toward Mark 290 (Wakker et al.\ 1996). begintableht begintabularlrrrr multispan5Table 1. Predicted optical depths , noalign line & Lambda & \log X_\odot^1 & \logDelta^2 & Tau , ZnII & 2026.1360 & -7.35 & 0.00 & 0.05 , ZnII & 2062.6640 & -7.35 & 0.00 & 0.10 , CrII & 2056.2540 & -6.32 & -0.40 & 0.06 , CrII & 2062.2340 & - 6.32 & -0.40 & 0.04 , CrII & 2066.1610 & -6.32 & -0.40 & 0.03 , MnII & 2576.8770 & -6.47 & -0.50 & 0.14 , MnII & 2594.4990 &-6.47 & -0.50 & 0.11 , MnII & 2606.4620 & -6.47 & -0.50 & 0.08 , FeII & 2249.8768 & -4.49 & -0.60 & 0.07 , FeII & 2260.7805 & -4.49 & -0.60 & 0.10 , FeII & 2344.2140 & -4.49 & - 0.60 & 3.06 , FeII & 2374.4612 & -4.49 & -0.60 & 0.80 , FeII & 2382.7650 & - 4.49 & -0.60 & 8.52 , FeII & 2586.6500 &-4.49 & -0.60 & 1.99 , FeII & 2600.1729 & -4.49 & -0.60 & 6.92 , hline noalign 1: X_\odot is solar abundance; 2: Delta is the depletion for halo gas; endtabular endtable The resulting predictions are summarized in Table 1. This shows that the column density of Fe II should be easily measurable, since the predicted optical depths span a factor of 100. We can combine the result with the measured column density of sulfur or zinc to derive the depletion of iron. Mn II will be measurable unless the depletion is much larger than expected for halo gas. For the important \ZnII\ lines low optical depths are predicted (0.05 and 0.10). The E230M echelle gives 5 km/s pixels and 13 km/s resolution. A 3-Sigma detection thus requires an S/N ratio of ~20 in the continuum. Table 9.1 of the \STIS manual shows that for a long exposure the first HST orbit requires 9 minutes of guide star acquisition, 8 minutes of target acquisition, and 5 minutes of peak-up time. Subsequent orbits need 6 minutes of re- acquisition time. Another 8 minutes of MAMA overhead is needed during the first orbit and 4 minutes of read-out and data-management overhead during every 60 minutes of exposure time, i.e.\ every orbit. At 57 degrees declination each orbit has a 57-minute usable window, so there is 27 minutes of on- source exposure time in the first orbit, 47 minutes in the next ones. Figure 3 shows an IUE low-dispersion spectrum of Mark 290, covering the full UV wavelength range. The flux in the near UV is about 1.8 \fu. We used the \STIS Exposure-Time Calculator to estimate that to reach an S/N of 20 at 2060 Angstrom, using the 0arcs2*0arcs2\ aperture, we need an exposure time of 565 minutes, or 12 orbits. With this exposure time, the S/N ratio at longer wavelengths will be >20, making the measurements of Fe and Mn easy. \medskip References \refer Anders E., Grevesse N., 1989 Geochim. Cosmochim. Acta 53, 197 \refer Savage B.D., Sembach K.R., 1996, ARAA 34, 279 \refer Verner D.A., Barthel P.D., Tytler D., 1994, A&AS, 108, 287 \refer Wakker B., van Woerden H., Schwarz U., Peletier R. Douglas N., 1996, A&A 306, L25 Real Time Justification ----------------------- None A major problem in converting absorption-line column densities to abundances can be that they are obtained along the (almost) infinitesimal pencil beams toward the background probes, while H I column densities are based on 21-cm data obtained with large radio beams (often >10'). Wakker & Schwarz (1991, A&A 250, 484) showed that in HVC cores N(H I) can vary by a factor five over a few arcminutes. Outside HVC cores the variations are less and 9'\ resolution Effelsberg spectra may suffice. A high-resolution (1') Westerbork map has been recently obtained for Mark 290, but it still needs to be fully analyzed. This consideration is important for the derivation of absolute abundances, which was the goal of our original \GHRS program to determine the value of S/H. For this program we are interested in abundance ratios, for which the \STIS data alone suffice. Calibration Justification ------------------------- Additional Comments ------------------- ------------------------------------------------------------------------------------ 8150( 4) - 02/05/99 16:16 - [ 3] Data Distribution Paper Products: NO Media: ELECTRONIC Blocking Factor: 10 Ship To: PI_Address Ship Via: Email: ------------------------------------------------------------------------------------ 8150( 4) - 02/05/99 16:16 - [ 4] TARGET LIST a) Fixed Targets ------------------------------------------------------------------------------------------------------------------------------------ Tar| Target | Target | Target |Coord | Radial | Flux data No | Name | Description | Position |Eqnx | Vel. | ------------------------------------------------------------------------------------------------------------------------------------ 1 MARK290 GALAXY, SEYFERT, ISM, RA = 15H 35M 52.5S +/- 0.1S, DEC J2000 Z=0.030 V = 14.96 HIGH VELOCITY CLOUD, = 57D 54' 09.6" +/- 0.2", PLATE- B-V = 0.60 INTERMEDIATE VELOCITY ID = 01OK U-B = -0.62 CLOUD F-CONT(2050)=1.8e-14 Comments: Continuum flux from IUE low-dispersion spectrum 8150( 4) - 02/05/99 16:16 - [ 5] Visit: 01 Visit Priority: Visit Requirements: On Hold Comments: Additional Comments: Exposures ------------------------------------------------------------------------------------------------------------------------------------ Exposure| Target |Instr | Oper. | Aper |Spectral|Central| Optional |Num| Time | Special Number | Name |Config| Mode |or FOV |Element |Waveln.| Parameters |Exp| | Requirements ------------------------------------------------------------------------------------------------------------------------------------ 1 MARK290 STIS/C ACQ F28X50L MIRROR 1 1.0 S ONBOARD ACQ FOR 2 CD P ------------------------------------------------------------------------------------------------------------------------------------ 2 MARK290 STIS/C ACQ/PEA 0.2X0.2 MIRROR 1 1.0 S ONBOARD ACQ FOR 3 CD K ------------------------------------------------------------------------------------------------------------------------------------ 3 MARK290 STIS/N ACCUM 0.2X0.2 E230M 2269 2 1100 S UV-MAM A ------------------------------------------------------------------------------------------------------------------------------------ 4 MARK290 STIS/N ACCUM 0.2X0.2 E230M 2269 6 1450 S UV-MAM A ------------------------------------------------------------------------------------------------------------------------------------ 8150( 4) - 02/05/99 16:16 - [ 6] Visit: 02 Visit Priority: Visit Requirements: On Hold Comments: Additional Comments: Exposures ------------------------------------------------------------------------------------------------------------------------------------ Exposure| Target |Instr | Oper. | Aper |Spectral|Central| Optional |Num| Time | Special Number | Name |Config| Mode |or FOV |Element |Waveln.| Parameters |Exp| | Requirements ------------------------------------------------------------------------------------------------------------------------------------ 1 MARK290 STIS/C ACQ F28X50L MIRROR 1 1.0 S ONBOARD ACQ FOR 2 CD P ------------------------------------------------------------------------------------------------------------------------------------ 2 MARK290 STIS/C ACQ/PEA 0.2X0.2 MIRROR 1 1.0 S ONBOARD ACQ FOR 3 CD K ------------------------------------------------------------------------------------------------------------------------------------ 3 MARK290 STIS/N ACCUM 0.2X0.2 E230M 2269 2 1100 S UV-MAM A ------------------------------------------------------------------------------------------------------------------------------------ 4 MARK290 STIS/N ACCUM 0.2X0.2 E230M 2269 6 1450 S UV-MAM A ------------------------------------------------------------------------------------------------------------------------------------ 8150( 4) - 02/05/99 16:16 - [ 7] Visit: 03 Visit Priority: Visit Requirements: On Hold Comments: Additional Comments: Exposures ------------------------------------------------------------------------------------------------------------------------------------ Exposure| Target |Instr | Oper. | Aper |Spectral|Central| Optional |Num| Time | Special Number | Name |Config| Mode |or FOV |Element |Waveln.| Parameters |Exp| | Requirements ------------------------------------------------------------------------------------------------------------------------------------ 1 MARK290 STIS/C ACQ F28X50L MIRROR 1 1.0 S ONBOARD ACQ FOR 2 CD P ------------------------------------------------------------------------------------------------------------------------------------ 2 MARK290 STIS/C ACQ/PEA 0.2X0.2 MIRROR 1 1.0 S ONBOARD ACQ FOR 3 CD K ------------------------------------------------------------------------------------------------------------------------------------ 3 MARK290 STIS/N ACCUM 0.2X0.2 E230M 2269 2 1100 S UV-MAM A ------------------------------------------------------------------------------------------------------------------------------------ 4 MARK290 STIS/N ACCUM 0.2X0.2 E230M 2269 6 1450 S UV-MAM A ------------------------------------------------------------------------------------------------------------------------------------ 8150( 4) - 02/05/99 16:16 - [ 8] Summary Form for Proposal 8150 Item Used in this proposal ------------------------------------------------------------------------------------------------------------------------------------ Apertures F28X50LP 0.2X0.2 ------------------------------------------------------------------------------------------------------------------------------------ Configurations STIS/CCD STIS/NUV-MAMA ------------------------------------------------------------------------------------------------------------------------------------ Opmodes ACQ ACQ/PEAK ACCUM ------------------------------------------------------------------------------------------------------------------------------------ Proposal Category GO ------------------------------------------------------------------------------------------------------------------------------------ Special Requirements ONBOARD ACQ FOR 2 ONBOARD ACQ FOR 3 ------------------------------------------------------------------------------------------------------------------------------------ Spectral Elements MIRROR E230M ------------------------------------------------------------------------------------------------------------------------------------ Target Names MARK290 ------------------------------------------------------------------------------------------------------------------------------------ Wavelengths 2269 ------------------------------------------------------------------------------------------------------------------------------------